There have been recently proposed processes for producing polyester multifilament yarns composed of fine filaments having a single filament fineness of 1 dtex or below utilizing high-speed spinning. For example, JP-A 56-123409 (hereunder, JP-A means “Japanese Unexamined Patent Application”) discloses “a process for producing a polyester fine multifilament yarn comprising continuously drawing a polyester undrawn yarn obtained by high-speed spinning and having a birefringence of 1×10−3 to 120×10−3, a shrinkage percentage in boiling water of 20 to 60% and a single filament fineness of 1.0 de (1.1 dtex) or below without winding the polyester undrawn yarn once at 1.05 to 1.6 times.” The polyester fine multifilament yarn obtained by the process is already drawn and cannot be subjected to frictional false twist texturing. Thereby, uses thereof are limited.
Furthermore, Japanese Patent Publication No. 3043414 discloses “a process for preparing a spin-oriented fine polyester multifilament yarn of denier in the range of about 1 to about 0.2 comprising melting a polyester polymer having a relative viscosity LRV in the range of about 13 to about 23, a zero-shear melting point in the range of about 240 to about 265° C. and a glass-transition temperature in the range of about 40 to about 80° C., then heating the polyester polymer to a temperature in the range of about 25 to about 55° C. above the melting temperature of the polymer at a residence time less than about 4 minutes, extruding the melt through a spinneret capillary at a mass flow rate in the range of about 0.07 to 0.7 g/min, a cross-sectional area in the range of about 125×10−6 to about 1250×10−6 cm2 and a length (L) and a diameter (D) such that the capillary length/capillary diameter ratio (L/D) is at least 1.25 and less than about 6, protecting the extruded melt from direct cooling as it emerges from the spinneret capillary over a distance in the range of at least 2 cm and less than about 12 dpf1/2 cm, cooling the extruded melt to below the glass-transition temperature and attenuating to an apparent spinline strain in the range of about 5.7 to about 7.6 and to an apparent internal spinline stress in the range of about 0.045 to about 0.195 g/d, then converging the cooled filaments into a multifilament bundle at a distance from the spinneret capillary in the range of about 50 to about 140 cm and winding up the multifilament bundle at a withdrawal speed in the range of about 2000 to about 6000 m/min.”
To be sure, when the melt spinning of the polyester is carried out in the range of the extremely limited conditions, the spin-oriented polyester fine multifilament yarn having a birefringence of about 0.03 to about 0.1 is obtained. The fine polyester multifilament yarn having the birefringence can be subjected to frictional draw-false twist texturing. However, even under the extremely limited spinning conditions, a phenomenon in which a molten polymer just after extrusion causes droplet breakage and results in yarn breakage as the polymer throughput is reduced tends to occur simply by preventing the melt from direct cooling in a specific distance range as the molten polymer emerges from the spinneret capillary. As a result, there are increasingly frequent cases where the stable spinning is difficult. In addition, when the polymer filaments are converged into the filament bundle at the distance from the spinneret capacity in the range of about 50 to about 140 cm, there remain problems that the running state of the extruded polymer filaments becomes unstable as the total number of single filaments is increased (especially in the case of 50 filaments/spinline or above), and the uniformity of the resulting spin-oriented fine multifilament yarn is lowered (evenness U % is increased).
On the other hand, soft hand and performances such as heat reserving properties, water and moisture absorptivity of the polyester fine false twist textured yarn having a single filament fineness of 1 dtex or below are improved as compared with those of a usual polyester false twist textured yarn when converted into a fabric. Therefore, the polyester fine false twist textured yarn has been widely used in clothes uses. For example, JP-A 4-194036 discloses a water absorbing fine false twist textured yarn which is a false twist textured yarn composed of polyester multifilaments having a single filament fineness of 0.7 denier (0.78 dtex) or below and having a limited cross section flatness coefficient and a limited total crimp ratio and a process for producing the yarn. JP-A 2002-038341 discloses a polyester false twist textured yarn composed of a polyester containing a metal-containing phosphorus compound and an alkaline earth metal compound and having a single filament fineness of 0.6 dtex or below, a limited flatness coefficient and a limited thermal stress peak value and an improved depth and sharpness of color when dyed and a process for producing the yarn.
Limited performances are surely improved in the fine polyester false twist textured yarn produced by such a special limited process. When simultaneous draw-false twist texturing of a usual undrawn polyester yarn, however, is carried out under the conditions, there are problems that the resulting false twist textured yarn cannot be used as a false twist textured yarn because yarn breakage frequently occurs or fluffs or non-untwisted spot unvennesses are frequently formed in the resulting false twist textured yarn with great quality unevenness such as uneven dyeing as the number of single filaments increases and the fineness becomes small.
Further, even in the field of fine polyester fibers, speed-up of weaving and knitting is promoted in order to improve the productivity and market demands for false twist textured yarns responsive to the speed-up have been increasing. Fly wastes are easily formed and there is a tendency to increase frequency of stopping weaving in a loom when even a false twist textured yarn of good quality with slight fluffs or non-untwisted spot unevennesses is unwound at a speed as high as 1200 m/min or above. A false twist textured yarn having more improved performances in a weaving process or a knitting process is desired.
Therefore, it is a first object of the present invention to provide a process for stably producing a frictional draw-false-twist texturable spin-oriented polyester fine multifilament yarn and the polyester fine multifilament yarn.
It is a second object of the present invention to provide a process for stably producing a polyester fine false twist textured yarn with slight fluffs, non-untwisted spot unevennesses and uneven dyeing in spite of an fine multifilament yarn having a small fineness and a large number of filaments by simultaneous draw-false twist texturing and the polyester fine false twist textured yarn.
It is a third object of the present invention to provide a process for stably producing a polyester fine false twist textured yarn with slight fluffs, non-untwisted spot unevennesses and uneven dyeing in spite of a small fineness and a large number of filaments, scarcely forming fly wastes even when unwound at a high speed and having good performances in a weaving or a knitting processes.